The present invention generally relates to voltage converters and more particularly to DC to DC voltage converters that are used for power hand tools and the like which have motors that operate with DC voltage or which otherwise have DC voltage source requirements.
Direct current to direct current (DCxe2x80x94DC) voltage converters are certainly known in the prior art and voltage converters that convert the voltage levels upwardly or downwardly are also known. Those which convert the voltage upwardly are also known as boost converters and those which convert voltage downwardly are known as buck converters. In the art of cordless power tools which use DC motors or motors that are capable of being operated with either DC or AC power, such as motors that are presently known as universal motors, there is a desire to increase the performance of tools in terms of the torque that is generated as well as the longevity of operation. A common method to increase cordless power tool performance is to increase the system operating voltage, which in the past has been achieved by simply adding more battery cells to drive bigger motors. However, this solution creates an obvious dilemma in that more cells require more space and weigh more which results in the whole tool becoming bigger and clumsier. This is quite undesirable with respect to some types of tools such as the small rotary hand tools marketed under the Dremel brand made by the S-B Power Tool Company of Chicago, Ill., for example. Such tools are relatively small in that they can be easily held in one hand by a user and they accept various kinds of drill bits, grinding bits, saw blades, sanding disks and the like which are used in hobby work, woodworking, maintenance and repair work as well as other tasks. There is a need to provide an improved DCxe2x80x94DC voltage converter that can adapt different voltage batteries to different motor voltages, whether the conversion is either up or down from a fixed voltage battery or battery pack.
The present invention is directed to a DCxe2x80x94DC voltage converter that is capable of increasing as well as decreasing the voltage of a battery pack for application to a power tool or other load. The converter thereby enables different voltage batteries to be used with power tools that have motors that operate at different voltages. The converter can therefore also be characterized as a universal adapter circuit where either up or down voltage conversion can be employed depending upon the particular application that is being implemented. The simplicity of the circuitry of various embodiments of the present invention permits a relatively small tool to hold the battery as well as the converter and the motor. In the event that a removable battery pack is used, the simplicity of the converter is such that it can be housed in either the battery pack or in the tool itself. The design has a major advantage in that the converter itself is very compact and is therefore extremely well suited for use in small hand tools such as the Dremel hand tool mentioned above. It is also possible to have a converter located in a battery pack with the converter being available to operate in an up as well as down voltage conversion.
Another major advantage is that when used in a small DC cordless power tool such as the Dremel tool, a higher voltage motor can be used with the converter battery voltage to operate the higher voltage motor without the necessity of increasing the number of cells to obtain the necessary higher voltage.
The voltage conversion is accomplished by using a relatively high speed switching circuit wherein varying the duty cycle of the switching circuitry varies the output voltage. An alternative embodiment utilizes feedback control circuitry to automatically adjust the duty cycle so that the speed of the motor, which is a function of the voltage applied to it, would be held generally constant.